Electric machines such as hydro generators have a rotor connected to a hydro turbine and a stator. Often the rotor and stator are assembled with a vertical axis.
The rotor comprises a shaft, a spider on the shaft and a laminated drum on the spider.
The spider defines a plurality of axial ducts and the laminated drum has a plurality of radial cooling channels connected to the ducts, such that during operation a gas (such as air) circulates through the ducts and cooling channels to cool the rotor and stator.
Air circulation causes losses that should be minimized.
In particular, losses depend on:                the total air volume flow circulating through the machine, and        the air volume flow distribution within the machine.        
In order to reduce the circulation losses, traditionally electric machines are designed in order to optimize air volume circulation; nevertheless when designing the cooling features of such electric machines some uncertainties exist that cannot be calculated in advance.